The invention relates to an electron multiplier element for secondary emission having a first metal plate which has at least one multiplier hole, having one input aperture and one output aperture and whose wall has emissive power. A second metal plate in parallel with the first plate has at least one auxiliary hole disposed opposite the output aperture of the multiplier hole. The second plate is electrically insulated from the first and brought to an electric potential which is higher than electric potential of the first plate.
The invention also relates to an electron multiplier comprising N multiplier elements as described , application of one electron multiplier in a photomultiplier tube and a method of manufacturing the electon multiplier element.
A particular advantageous use of the invention is in the field of photomultiplier tubes.
A multiplier element of the type described in the opening paragraph is known from French Patent Application No. 2,549,288 to which U.S. Pat. No. 4,649,314 corresponds. This application describes a multiplier element whose multiplier holes are either symmetrical, that is to say, the input and output apertures are coaxial, or asymmetrical, that is to say, the input and output apertures are shifted with respect to one another, whilst the output aperture is located opposite the input aperture. This electron multiplier element structure has the drawback of a limited collection efficiency because numerous incident electrons can traverse the multiplier element without undergoing multiplication on the walls of the multiplier holes by passing directly through the input and output apertures. On the other hand this loss of collection efficiency reoccurs at each stage of a multiplier comprising N multiplier elements of the known type and is thus translated into a loss of gain, a linearity error and a longer response time, for example, when this multiplier is incorporated in a photomultiplier tube.
According to the invention the input apertures are longer than respective output apertures, and a perpendicular projection of the output aperture on to a plane parallel to the plate lies at least partly and preferably entirely outside the corresponding projection of the input aperture.
Thus, the majority of incident electrons reaching the electron multiplier element, with the exception of the few electrons occurring at an angle of incidence which is too large, encounter the wall of the multiplier hole where they are subjected to a multiplication. Tests carried out on the multiplier holes having entirely shifted apertures have shown that the collection efficiency of such a multiplier element is substatially improved. Despite the relatively large dimensions of the multiplier hole, multiplied electrons do not return to the wall of the hole where they would be lost. This experimental fact sustains the idea of a possibility of electrons rebounding without any loss on the wall of the multiplier hole.
In a general embodiment of the multiplier element the first metal plate has a plurality of multiplier holes arranged in a regular plane network which may be square-shaped or hexagonal, whilst the input and output apertures are circular, square-shaped or hexagonal.
In an electron multiplier comprising N multiplier elements according to the invention the second metal plate of the (i)th multiplier element is brought to an electric potential which is identical to the electic potential of the first metal plate of the (i+1)st multiplier element.
In this manner a better collection of electrons is ensured between a multiplier element and the next element when these elements are relatively remote from each other. With an improved collection efficiency the electron multiplier also provides the possibility of forming an image. Two geometry types may be envisaged, one in which the N multiplier elements are arranged in a parallel configuration with respect to one another and in another advantageous geometry the N multiplier elements are arranged in a head-to-tail configuration with respect to one another which at a each multiplication permits the electron beam to retrace.
The multiplier can be used advantageously in a photomultiplier tube comprising a photocathode and n adjacent anodes. The multiplier is placed in the proximity of the photocathode and is divided into n secondary multipliers by partitions which are impervious to electrons and are located substantially opposite separation zones of two adjacent anodes in such a manner that n secondary photomultiplier tubes are obtained in the same photomultiplier tube.
Finally a method of manufacturing a first metal plate of an electron multiplier element according to the invention is characterized in that the two faces of the same metal plate are simultaneously etched with the aid of a pair of masks whose successive windows increase in size and are shifted with respect to one another, the windows of the last pair of masks reproducing the shapes of the input aperture and the output aperture, respectively, of the multiplier hole.